The present invention relates to a system for transmitting drive between the engine crankshaft of an internal combustion engine of a motor vehicle and a group of auxiliary devices provided with respective drive shafts and including an electric machine operable to function selectively as a generator and a motor.
More specifically, the subject of the invention is a transmission system of the type comprising                at least one belt acting to couple the drive shafts of the said auxiliary devices with a pulley operatively connectable to the crankshaft of the internal combustion engine, and        a servo controlled clutch operable selectively to control coupling of the said pulley with the crankshaft of the internal combustion engine.        
A drive transmission system of this type is known, with an electromagnetically controlled clutch which is normally excited and engaged to couple the said pulley to the crankshaft for rotation with the internal combustion engine. This known system makes it possible to achieve the following functions or operative conditions:                activation of the auxiliary devices (compressor of the air conditioning system, electric machine functioning as generator etc) whilst the internal combustion engine is running; the electromagnetically controlled clutch is excited and engaged;        activation of the auxiliary devices, and in particular of the compressor of the air conditioning system, whilst the internal combustion engine is not running; in this case the electromagnetically controlled clutch is de-excited, and therefore disengaged, and the said electric machine is made to operate as a motor to drive the compressor in rotation by means of the said transmission belt; and        re-starting of the internal combustion engine (hot) by means of the said electric machine, by operating as a motor, for example to achieve a so-called stop-start function; in this case the electromagnetically controlled clutch is excited and engaged to couple the said electric machine to the crankshaft of the internal combustion engine in the re-starting phase.        
The known system described above has the disadvantage of requiring that the electromagnetic clutch remains permanently energised when the internal combustion engine is running. The prolonged energisation of this clutch involves a high consumption of electrical energy which negatively influences the overall energy balance.